Light-Harvesting Heptadecameric Porphyrin Assemblies

Abstract

New porphyrin assemblies containing 17 porphyrin molecules are constructed by using free base TPP-type porphyrins having eight pyrazine moieties 1. Spectroscopic titration of dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II) 2 with these porphyrins shows that the processes of the formation of the heptadecameric porphyrin assemblies may be analyzed as eight independent equilibrium processes with an identical binding constant. All binding constants are larger than 5 x 107 M-1 which is the determinable upper limit of the present titration method. In all cases, the fluorescence spectrum of the 1:8 mixture of 1 and 2 consists of the major fluorescence of 1 and the minor one of 2.pyrazine complex even in the presence of the large excess of the antenna pigments. The observed spectra are well reconstructed by the form of faF1 + fbF2, where F1 and F2 are the fluorescence of 1 and the 2.pyrazine complex measured separately at the corresponding concentrations. Interestingly, the general trend that values of fa are nearly equal to those of r564 x (1 - fb) in all cases is found, where r564 is the absorption ratios of the 2.pyrazine moiety and the central free base porphyrin in the assemblies at 564 nm. The observation indicates the excitation of the central porphyrin is directly enhanced by the absorption of the antenna pigments even in such large scale assemblies. Thus, the antenna effect for 1 having largest r564 results in 77 times fluorescence enhancement of the central free base porphyrin. The systems also show interesting dependency of energy-transfer efficiencies on the topological arrangement of the antenna elements.

About Supramolecular Assemblies of π-Conjugated Systems

“…It illustrates that although the processability and mechanical robustness of block copolymers may be superior to those of analogous oligomers, the degree of self-assembled order found in oligomer-based systems may be lost in alternating polymers. 183 Oligo(ethylene oxide) can also be used in alternation with distyrylbenzene units (68), providing efficient light-emitting diodes, presumably by efficient ion-conducting pathways. 184,185 Polymers consisting of alternating perylene bisimide chromophores and flexible polytetrahydrofuran segments of different length (69) have been studied using absorption and (time-resolved) photoluminescence spectroscopy.…”

Supramolecular Organization of Porphyrins and Phthalocyanines by Use of Biomimetic Coordination Methodology

“…Kuroda and coworkers prepared pyrazine-appended free-base porphyrin 98 as a multiple binding site for porphyrin tetrabenzoic acid 99, which forms dimer 99 2 with quadruple hydrogen bonding interaction. 147,148 Photoexcitation energy of peripheral Zn-porphyrin was transferred with 61% efficiency, resulting in fluorescence emission from the central free-base porphyrin 98 core (Scheme 48).…”

Challenges and breakthroughs in recent research on self-assembly

“…Kuroda and coworkers reported porphyrin assemblies containing 17 porphyrin molecules [393]. In their system, the photo-energy absorbed by the entire system could be effectively converted to emission by the single central porphyrin.…”